The reason why a sequel took so long to be released was because Sony wanted to film "The Lost Symbol," but the production of that adaptation became too complicated as Ron Howard preferred to only produce it rather than directing, and Tom Hanks would not be returning as Professor Robert Langdon. In the end, the production took so long that the novel Inferno was released in 2013, and Sony decided to produce that adaptation instead, for which most of the crew of the previous films returned, including Howard as director and Hanks reprising his role as the main character. The Lost Symbol was later filmed as a prequel TV mini series, The Lost Symbol (2021).
During Zobrist's video talk at the start of the film, he describes "Bartlett's Beaker". Albert Bartlett (1923-2013), a long-time professor at the University of Colorado, gave the talk "Arithmetic, Population, and Energy" over 1700 times. He said "the greatest shortcoming of the human race is our inability to understand the exponential function."
None of the movies in the Robert Langdon saga have been produced in the order that their corresponding novels by Dan Brown were released. The Da Vinci Code (2006) was made first, but adapted from Brown's second book; Angels & Demons (2009) was the second movie to be released, but adapted from the first book. "Inferno" is the third film to be adapted, but from the fourth book in the series. The third book, "The Lost Symbol", was initially skipped over because the studio preferred to adapt "Inferno", but was later adapted as the fourth project, the prequel TV series The Lost Symbol (2021).
"Inferno" had a budget of $75 million, which was much lower than the first two installments.